a, Hệ PT \(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{3y+6}{4}\\-\dfrac{5\left(3y+6\right)}{4}+ay=8\end{matrix}\right.\)
- Từ PT ( II ) \(\Rightarrow-\dfrac{15y}{4}-\dfrac{15}{2}+ay=8\)
\(\Leftrightarrow y\left(a-\dfrac{15}{4}\right)=\dfrac{31}{2}\)
\(\Leftrightarrow y=\dfrac{\dfrac{31}{2}}{a-\dfrac{15}{4}}=\dfrac{15,5}{\dfrac{1}{4}\left(4a-15\right)}=\dfrac{62}{4a-15}\)
- Thay lại y vào PT ( I ) ta được : \(x=\dfrac{3\left(\dfrac{62}{4a-15}\right)+6}{4}\)
\(=\dfrac{\dfrac{186+6\left(4a-15\right)}{4a-15}}{4}=\dfrac{186+24a-90}{16a-60}=\dfrac{24a+96}{16a-60}=\dfrac{6a+24}{4a-15}\)
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b, - Để hệ phương trình có nghiệm âm :\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{6a+24}{4a-15}< 0\\\dfrac{62}{4a-15}< 0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}6a+24>0\\4a-15< 0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}a>-4\\a< \dfrac{15}{4}\end{matrix}\right.\)
\(\Rightarrow-4< a< \dfrac{15}{4}\)
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